The biochemical and biological characteristics of bovine brain and lung tissue thromboplastins and platelet factor 3 are being evaluated. These lipoprotein procoagulants are involved in the initiation of coagulation. The procoagulant activity resides in a phospholipid-dependent-protein component of these materials and is also found in human placental tissue. The protein moieties of these procoagulants are being spearated from the phospholipids by organic solvent (ethanol, sec-butyl alcohol) and detergent (deoxycholate, SDS) extraction and filtration procedures respectively. The isolates have been partially purified and characterized as to molecular weight, amino acid composition and carbohydrate content. The aim is to achieve a level of procagulatnt purity which will permit accurate comparison of similar components from different tissues (eg. Brain, lung, platelets, placenta, etc.) and to permit primary structural studies. The primary amino acid sequence of human platelet anti-heparin, platelet factor 4 (PF 4), has been determined. The bovine analogue is slightly larger and is currently being studied to determine the degree of homology between its structure and that of the human product. The human product is being evaluated in the intact and chemically modified form to help define the mechanism(s) of heparin binding and to ascertain the minimal molecular structure required for maximal heparin binding.